How can you be sure that God doesn't exist?

[BryanS]

 Why do you believe that God doesn't exist?
 How can you be sure that God's existence is not real, for certain?
 Humanity has still not uncovered a great deal about our own existence, or what's on the other side of the universe.

For your first question, the reason is that I have not seen any evidence justifying the existence of god. For one thing, there are so many gods that humans have worshiped over history, that there is no agreement on what this god is to go looking for it. And in the case of the Judeo Christian god, it is defined in such a way as to be unobservable.  In fact, there have been so many human invented gods that to take on the task of disproving each and everyone of them would be an impossible thing to humanly do. Which leads naturally to your second question.
Your second question conveniently shifts the burden of proof to the atheist. You almost certainly agree that any of thousands of other gods ever conceived of --other than the Judeo Christian one--had to have been human inventions. Since human have such a proclivity to invent gods, and since the concept of a god is itself a human invention, I will not accept the proposition that a god exists without some positive evidence for it. That is not the same thing as absolute proof of the non-existence of god, but the position I take is logically no different than refusing to accept the existence of Santa Clause or the Toothfairy as they are likewise inventions of the human imagination. So I do not put the level of certainty at 100%, but I do put it at very unlikely that any god exists. I would be open to changing my mind on the existence of god if presented with evidence, but I do not expect to see such evidence. Which leads naturally to your third question.
For much of human history, god was the explanation for the inexplicable, a "god of the gaps" as atheists are wont to call this idea. Since humanity has developed the scientific method of investigating the natural world, the gaps have only diminished. We have not to date required a god hypothesis to explain the natural world around us. In fact, phenomena attributed to god has continued to find alternate, scientific explanations. The god of the gaps is a diminishing entity and will only continue to diminish.  We may, at some point, discover evidence for this god. But so far any evidence for an existence of god has proven illusive.